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Geologic Time Study Guide
Adaptations and Natural Selection
Populations in a particular environment that are better adapted to living conditions there are
more likely to survive and reproduce offspring with those traits.
 There are genetic variations among species of similar populations.
 Members of a species differ from one another in many of their traits.
 An adaptation is a trait or behavior that helps an organism survive and reproduce.
Natural selection is the process that explains how survival changes species over time.
 For example:
o certain traits or adaptations involving color, camouflage, food gathering (beaks,
claws) and other physical traits
o sensory abilities
o behaviors that enhance the survival of a species.
A species is extinct if no members of that species are still alive.
 Most organisms that have ever lived on Earth are now extinct.
 Natural factors can cause extinctions such as has happened throughout Earth history.
o Examples include changes due to:
 volcanic eruptions and global warming
 global cooling during ice ages
 changes in oxygen levels in seawater
 a massive impact from an asteroid or comet
o Natural extinctions have occurred throughout geologic history.
 Not all extinctions have been negative because extinctions often clear the way for new
kinds of life.
 Man-made factors have caused extinctions in more recent times
o Examples include:
 the cutting of the rainforest regions
 removing natural habitats
 over-harvesting
 pollution.
o Many plants and animals are likely to become extinct in the near future if humans
do not make changes in way they are interacting with the Earth
o Human effects on the environment could threaten some biological resources that
humans may need.
 Species that have recently become extinct or that are endangered can be studied to
discover what natural or man-made survival resources caused the extinction or
endangerment and what could have been or could be done to prevent it from happening.
Geologic Time
The geologic time scale divides Earth’s long history
into units of time:
 eons are divided into eras
 eras are divided into periods
 periods can be further divided into epochs
Major information found on the geologic time scale
includes:
 Precambrian is the name given to the earliest
span of time in Earth history.
 Geologists divide the time between
Precambrian and the present into three long
units called eras (Paleozoic, Mesozoic,
Cenozoic).
o Eras end with a mass extinction
 Cambrian being the first period is important.
 With a more complete fossil record available,
the periods of the Cenozoic era are
subdivided further into epochs.
 Present day Earth is in the Cenozoic era and
the Quaternary period in the Holocene epoch.
 Geologic time has not ended.
In the study of the geologic time scale, the changes in life forms are often accompanied by
changes in environmental conditions on Earth.
 These environmental changes include:
o Impact of an asteroid or comet
 Earth’s atmosphere protects the planet from many of the meteors that
enter it, resulting in their burning up before striking the surface.
 At the end of the Mesozoic Era, when reptiles, early birds and mammals
thrived, many groups of animals disappeared suddenly.
 Scientists hypothesize that possibly a large asteroid or comet impacted
with Earth.
 This impact caused dust and smoke to rise into the atmosphere and
cause climatic changes, as well as the dying of many forms of plant life
and animals that depended on those plants for food.
 A major life form that disappeared at this time was the dinosaur.
o Climatic changes
 Earth’s environments have many different climates even today.
 Climate is an ever-changing condition on Earth.
 Earliest life forms were influenced by the climates produced by the
forming atmosphere and oceans of Earth.
 Life on land developed and flourished in the tropical climates and warm
shallow seas during the Paleozoic Era.
 Throughout this era as different land environments formed and
sea levels changed, new life forms developed.


Other life forms that could not adapt or find suitable conditions,
especially many marine species, disappeared.
 During the Mesozoic era, many climate changes occurred due to plate
tectonics and the movement of landmasses.
 Plants and animals that survived through this time had structures
and systems that allowed for greater adaptations, such as seed
coverings for plant seeds and protective body coverings or
constant internal temperature for animals.
 During the present Cenozoic era, climate conditions continue to change.
 Major ice ages caused the climate to become much cooler as ice
sheets and glaciers covered many areas of Earth.
 Many mountain ranges formed causing climate differences due to
elevation and due to location near those ranges.
o Volcanic activity
 From the earliest days while Earth was forming to present day, volcanic
activity has been part of the nature of this changing planet.
 During the Precambrian time volcanic activity was one of the most natural
events, but lava flows, ash clouds in the atmosphere, and heat made
conditions for life forms extremely difficult.
 Those simple life forms often did not survive these conditions.
 As continent collided and mountains built up due to plate tectonics,
volcanoes also formed.
 Volcanic activity continued to be common in the Paleozoic era.
During the rapid movement of plates in the Mesozoic era,
collisions and subduction produced extensive volcanic activity
around plate boundaries.
 Plate boundaries are still the location of much of Earth’s volcanic
activity.
 Very explosive volcanic activity can send ash and dust high into the
atmosphere where it is carried great distances around the Earth.
 The Sun can be blocked for long periods of time.
The eras of Earth history can be studied in light of conditions on Earth, the effect of
those conditions on life-forms, and the rapid changes to both due to catastrophes.
The geologic time scale is a record of the major events and diversity of life forms present in
Earth’s history.
 The geologic time scale began when Earth was formed and goes on until the present.
 At the end of each era a mass extinction occurred
o 90+% of organisms died out
o There were other minor extinctions going on during each period of geologic time.
 Using the fossil record, paleontologists have created a picture of the different types of
common organisms in each geologic period.
Paleozoic Era Life
 Paleozoic life began with the early invertebrates, such as trilobites and brachiopods
 Continued to develop early vertebrate fish
 Then came arachnids and insects
 Later came the first amphibians
 Near the era’s end the reptiles became dominant.
 Early land plants included simple mosses, ferns, and then cone-bearing plants.

The mass extinction that ended the era caused most marine invertebrates as well as
most amphibians to disappear.
Mesozoic Era Life
 Reptiles were the dominant animals of this era, including the various dinosaurs.
 Small mammals and birds also appeared.
 Toward the end of the era, flowering plants appeared
 The mass extinction that ended the era caused the dinosaurs to become extinct.
Cenozoic Era Life
 New mammals appeared while others became extinct.
 The diversity of life forms increased.
 Flowering plants became most common.
 Humans are also part of the most recent period of this era.
Relative Ages and Fossils
The relative age means the age of one object compared to the age of another object.
 Relative age does not tell the exact age of an object.
 The relative age of rocks and fossils can be determined using two basic methods:
ordering of rock layers and index fossils
 Ordering of Rock Layers
o Scientists read the rock layers knowing that each layer is
deposited on top of other layers.
o The law of superposition states that each rock layer is older
than the one above it.
 So using this layering, the relative age of the rock or
fossil in the rock is older if farther down in the rock
layers.
o Relative dating can be used only when the rock layers have
been preserved in their original sequence.
 Index Fossils
o Index fossils can be used to help find the relative age of rock layers.
o To be an index fossil –
 an organism must have lived only during a short part of Earth’s history;
 many fossils of the organism must be found in rock layers;
 the fossil must be found over a wide area of Earth;
 the organism must be unique.
o The shorter time period a species lived, the better an index it is.
o A key example of an organism used as an index fossil are trilobites
 Trilobites are a group of hard-shelled animals
whose body had three sections, lived in
shallow seas, and became extinct about 245
million years ago.
 If a trilobite is found in a particular rock layer,
it can be compared with trilobites from other
layers to estimate the age of the layer in
which it was found.
 Fossils that are found in many rock layers, therefore living long periods of time, do not
qualify as index fossils.
o A fossil is the preserved remains or traces of an organism that lived in the past,
usually more that 10,000 years ago.
o Fossils give clues to the diversity of living things over the history of Earth, give
clues to past climate and surface changes on Earth, and give clues to changes
that have occurred with organisms over time.
o There are different types of fossils based on how they were formed.
 Mold fossil – forms when sediments bury an organism and the sediments
change into rock; the organism decays leaving a cavity in the shape of
the organism.
 Cast fossil – forms when a mold is filled with sand or mud that hardens
into the shape of the organism.
 Petrified fossil (permineralized fossil) – forms when minerals soak into the
buried remains, replacing the remains, and changing them into rock.
 Preserved fossil – forms when entire organisms or parts of organisms are
prevented from decaying by being trapped in rock, ice, tar, or amber.


Carbonized fossil – forms when organisms or parts, like leaves, stems,
flowers, fish, are pressed between layers of soft mud or clay that hardens
squeezing almost all the decaying organism away leaving the carbon
imprint in the rock.
Trace fossil – forms when the mud or sand hardens to stone where a
footprint, trail, or burrow of an organism was left behind.